Structural-Electromagnetic Simulation Coupling and Conformal Antenna Design Tool By © 2018 Pedro Martin Mendoza Strilchuk B.S., University of Kansas, 2015 Submitted to the graduate degree program in Aerospace Engineering and the Graduate Faculty of the University of Kansas in partial fulfillment of the requirements for the degree of Master of Science. Chair: Dr. Emily J. Arnold Dr. Richard D. Hale Dr. Ronald M. Barrett-Gonzales Date Defended: March 15, 2018 ii The thesis committee for Pedro Martin Mendoza Strilchuk certifies that this is the approved version of the following thesis: Structural-Electromagnetic Simulation Coupling and Conformal Antenna Design Tool Chair: Dr. Emily J. Arnold Date Approved: April 26, 2018 iii Abstract Airborne and spaceborne radar has long been an effective tool for remote sensing, surveillance, and reconnaissance. Most airborne systems utilize antenna arrays that are installed inside the moldline of the aircraft or in radomes that protect the array from in-flight loads. While externally-mounted arrays can offer the advantage of larger apertures, sensor-vehicle interactions often result in performance degradation of both systems. The presence of an externally-mounted array will increase the vehicle’s drag and potentially affect the dynamics and control of the vehicle. In addition, in-flight structural loads will deform the array, thus resulting in relative phase errors. While there exist a multitude of physics-based simulation tools to determine the effects of the array on the aircraft performance, existing tools are not sufficient for generating deformed arrays necessary for determining in-flight array performance. In response to this need, a computer tool for analyzing antennas undergoing structural loads is developed. The Antenna Deformation Tool (ADT) has two primary uses: generating deformed geometry from the output of a structural Finite Element Model (FEM) for use in an Electromagnetic (EM) simulation, and designing conformal antenna arrays. The two commercial software packages ADT is optimized for are MSC NASTRAN and ANSYS HFSS. Specifically, ADT is designed to generate a deformed 3D Computer Aided Design (CAD) model from a NASTRAN structural mesh. The resulting CAD model is compatible with HFSS electromagnetic simulation software for the assessment of the effects of loads on performance. The main purpose for the development of ADT is to facilitate studies of how structural deformations affect airborne antenna arrays performance and to provide the capability to perform studies easily and quickly iv using different antennas on the same structural model. ADT capabilities are demonstrated using several representative airborne antenna array structures. ADT is also demonstrated in the design of conformal antenna arrays. ADT can import CAD geometry and deform it according to a prescribed deformation field. The deformation field can either be determined from structural simulations or be provided by the user. This functionality allows the user to take an existing planar antenna design and conform it to a desired shape. Within the scope of airborne antenna arrays, this would allow an engineer to conform the antenna to the moldline of the aircraft or other support structure. Currently, ADT can interpret only quad and triangular 2D elements from NASTRAN. In addition, its ability to interpret a surface from a point cloud is limited to surface meshes in which there are exactly four explicit vertices, or surfaces which have a fairly even boundary with no major discontinuities and can be divided into four even segments. ADT is tested on NASTRAN structural models of small to medium complexity, and the geometry generated from simple models is used in HFSS simulations with success (with occasional post processing required). The antenna deformation submodule shows favorable performance with sheet and solid CAD geometry, though post-processing is required in the case of the latter. Results of some deformed antennas simulated with HFSS in the 200 MHz range are presented. The surface error of the geometry produced by ADT varies with the type of input mesh, with curved meshes and surfaces having higher errors. In terms of average element edge length, the maximum surface error is up to 1% for surfaces with no to small curvatures, and up to 3.6% for highly curved surfaces. This translates to about 0.17% of the mesh diagonal. ADT contains a set of classes and functions which provide ample capabilities for surface generation from meshes, and the process implemented is mostly automatic, requiring minimal user intervention. v Due to ADT defining deformed geometry purely on separate meshes, adjacent surfaces are not associative and continuity between them is not guaranteed, which inherently can result in small intersections. These intersections can cause meshing problems with HFSS; however, these issues can be mitigated by adding a small offset. While demonstrated applications are still limited, ADT promises to substantially contribute to the design of aircraft-integrated antennas and multifunctional structures. With very limited capabilities for generating and assessing deformed antenna geometry currently existing, ADT represents a unique tool. ADT could be used not only in developing the next-generation of airborne remote sensing technologies, but to characterize in-flight performance of existing systems as well. vi Acknowledgments Completing my degree, let alone this thesis, would not have been possible without the help of many people, who I would like to thank. I would like to thank my family for supporting me. My mother and father, for encouraging me to pursue my goals, despite the emotional and material burdens and physical distancing this has caused. My brother and sister, for reminding me of the simple joys of life. Elizabeth, for being my “thesis buddy”, and her family for taking me under their wing in this strange new land. My aunt and grandfather for teaching me life lessons with their wisdom and supporting me throughout my entire life. And to my many other aunts, uncles, cousins and friends, who, to my fortune, are too many to list, and all of whom have helped me surmount obstacles through my life. Additionally, I would like to thank the University of Kansas and the Institute of International Education for giving me the opportunity to study the field I am passionate about, and providing me with material support. In particular, I would like to thank the department of Aerospace Engineering and my committee members for their commitment. Finally, I would like to thank my advisor, Dr. Emily Arnold, for being exceptionally supportive, patient, kind, and thoughtful before and throughout the completion of my MS degree. Getting to where I am now would have been all but impossible without her guidance and support. vii Dedication This thesis is dedicated to the memory of my beloved grandmother Anna, who passed away during the final stages of working on this thesis. Her unbounded kindness, eternal cheer, relentless work ethic, and utter unwillingness to yield in the face of adversity have been and continue to be my main inspiration. She has taught me many things – among them the importance of remembering one’s roots, no matter where in this wide world or in what condition one ends up. Even though life has taken me far from home, thanks to her, I always carry a piece of it in my heart. Bucovină, plai cu flori, Unde sunt ai tăi feciori? Au fost duşi în altă ţară, Dar se-ntorc la primavară. Şi-napoi când or veni Tot pe tine te-or iubi Romanian Folk Song viii Table of Contents Chapter 1: Introduction ................................................................................................................... 1 1.1 Motivation ........................................................................................................................ 2 1.2 Problem Description ......................................................................................................... 7 1.3 Thesis Organization.......................................................................................................... 9 Chapter 2: Previous Work ............................................................................................................. 11 2.1 Estimating Effects of Deformation on Antenna Arrays ...................................................... 11 2.2 Multiphysics Software ........................................................................................................ 15 2.3 Geometric Conversion Software ......................................................................................... 16 Chapter 3: Program Description ................................................................................................... 19 3.1 The Finite Element Method ................................................................................................ 19 3.1.1 Structural FEA NASTRAN ......................................................................................... 20 3.1.2 Electromagnetic FEA HFSS ........................................................................................ 21 3.2 Software Structure and Requirements ................................................................................ 23 3.2.1 Core module ................................................................................................................. 25 3.2.2 Transitional text file ..................................................................................................... 28 3.2.3 CAD export